Conventionally, a cationic photopolymerizable epoxy resin composition is used as one type of an adhesive used for mounting an electronic component such as an IC chip on a wiring substrate. Such a cationic photopolymerizable epoxy resin composition contains a cationic photopolymerization initiator that generates protons through light to initiate cationic polymerization. One known example of the cationic photopolymerization initiator is a sulfonium antimonate complex.
However, such a sulfonium antimonate complex contains, as a counter anion, SbF6− in which fluorine atoms are bonded to antimony, which is a metal. Therefore, a large amount of fluorine ions are generated during cationic polymerization. This induces migration between different metals, causing a problem of corrosion of metal wiring and connection pads. Patent Document 1 proposes to use, as a cationic polymerization initiator, a sulfonium borate complex that uses, instead of SbF6−, a tetrakis(pentafluorophenyl)borate anion [(C6F5)4B−] in which fluorine atoms are bonded to carbon atoms. A complex of the following formula (1c) [p-hydroxyphenyl-benzyl-methylsulfonium tetrakis(pentafluorophenyl)borate] is actually commercially available.

When an electronic component is mounted on a wiring substrate, the bonding area often cannot be irradiated with light. Therefore, an attempt is being made to use the specific sulfonium borate complex disclosed in Examples of Patent Document 1 as a thermal cationic polymerization initiator for a thermal cationic polymerizable epoxy resin composition. In this case, there is a demand not only to reduce the amount of fluorine ions generated during cationic polymerization to improve the electrolytic corrosion resistance of the epoxy resin composition but also to improve the low-temperature rapid curability of the epoxy resin composition to improve productivity.    [Patent Document 1] Japanese Patent Application Laid-Open No. Hei. 09-176112.